Food Tests Required Practical 4

• What is the aim of the food tests described?

  To carry out qualitative chemical tests for starch, reducing sugars, proteins, and lipids.
• What equipment is needed for the iodine test for starch?

  • Food sample
  • A test tube
  • Iodine solution (0.1 molar)
  • Pipettes
• What happens to the solution in the iodine test if starch is present?

  The solution turns from brown to blue-black.
• What is the first step in the iodine test for starch?

  Put some of the food sample into a test tube.
• What are the steps in the Benedict’s test for reducing sugars?

  1. Add an equal volume or excess of Benedict’s solution to the food sample in a test tube.
  2. Place in a hot water bath for a few minutes.
  3. Observe for a brick red precipitate or a blue solution.
• What indicates the presence of reducing sugars in the Benedict's test?

  A brick red precipitate is formed.
• What equipment is needed for the Benedict's test?

  Food sample, a test tube, Benedict’s solution, hot water bath, thermometer, and pipettes.
• What is the method for testing proteins using Biuret solution?

  1. Add a few drops of Biuret’s reagent to the food sample in a test tube.
  2. Shake the solution to mix and wait for a few minutes.
  3. Observe for a color change from blue to purple.
• What color change indicates the presence of protein in the Biuret test?

  The solution turns from blue to purple.
• What equipment is needed for the protein test?

  • A test tube
  • A 10cm³ measuring cylinder
  • Biuret solution
• What are the steps in the test for lipids?

  1. Add a few cm³ of ethanol to the food sample.
  2. Pour the mixture into a test tube of equal volumes of distilled water.
  3. Observe for a white emulsion on the surface.
• What indicates the presence of lipids in the lipid test?

  A white emulsion is formed on the surface of the mixture.
• What is the name of the test used to detect lipids?

  The emulsion test.
• What are the sources of error in the food tests?

  • Colour change in Benedict’s and Biuret tests may be subtle.
  • Difficult to judge if the concentration of the tested molecule is low.
• What safety precautions should be taken during the food tests?

  • Tie hair back and wear safety goggles.
  • Handle Biuret solution with care due to its poisonous and corrosive nature.
  • Keep ethanol solution away from flames as it is highly flammable.